Acoustic device



Aug. 18, 1942. w. R. HARRY 2,293258 I ACOUSTIC DEVICE I Filed Nov. 24,19:59 '2 Sheets-Sheet 1 F/GQ/ PRESJ'URE GRADIENT ELEMENT 1 I PRESSUREELEMENT 9 J0 J1 J2 27 24 m VENTOR W R. HA RR Y Y Maw/ aw.

' ATTORNEY CONDENSER TRANS! TTER Aug. 18, 1942. Q w HARRY 2,293,258

ACOUSTIC DEVICE v Filed Nov. 24, 1939 2 sheat s-Sheet 2 M aaD/G.

' ATTORNEY no lso mo /N VE N W R. HA V Patented Aug. 18,1942

UNITED STATES PATENT OFFICE,

ACOUSTIC DEVICE William R. Harry, New. York, N. Y.,' assignor to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation 0!New Yorlr Application November 24, 1939, Serial No. 305,792; 1 Claim.(o1. 179-1) 3 This invention relates to acoustic devices and moreparticularly to sound pick-up or transmitter devices havingunidirectional response characteristics. a 1

Sound pick-up or transmitter devices, that are ordinarily designated asunidirectional, have maximum sensitivity to sound from one direction anda much lower sensitivity to sound from the opposite direction. Some ofthe known unidirectional transmitters comprise means responsive to boththe pressure and the pressure gradient of impinging sound waves, wherebythe response to sound from directly in front is maximum and to soundfrom directly in the rear is zero. The response to soundfrom pointsintermediate these two extremes is such that the curve indicative of.the response of the transmitter, determined by a plot of responseagainst angle, is a cardioid. Such transmitters may be said to have acardioid response characteristic or a cardioid sensi-,

'tivity'pattern.

For convenience of description the direction for which the sensitivityis maximum may be designated as the 0 degree position or direction andthat for which the sensitivity is zero as the 180 degree position ordirection. Furthermore, since the sensitivity pattern is a figure ofrevolution about the 0 degree-180 degree axis, consideration of a planethrough this axis is sufficient for most purposes. at the front, thatis, between the 0 degree and 90degree and the 0 degree and 270 degreepositions, and low at the rear or between the 180 degree and 90 degreepositions, and the 180 degree and 270 degree positions. Moreover, theresponse is very low over a given angle less than 90 degrees on eitherside of the 180 degree position. A substantially true cardioid responsecharacteristic may be obtained byproper adjustment and proportioning ofthe elements of the pick-up device. Transmitters of the type designatedin the foregoing are characterized by a directivity index that issubstantially 1/3. The directivity index which is a measure of thedirectionality may be defined as the ratio between transmittersensitivity to random sounds and to sounds from the front or directionof maximum response. As will be obvious from the foregoing definition ofdirectivity index, an increase in the denominator of the fractionindicating the value oi said index indicates improved directionality.For example, the directivity indices for a non- In such a plane, theresponse is highdirectional transmitter, a bidirectional trans mitterand a unidirectional transmitter may be One object of this invention isthe production. of unidirectional sound pick-up devices having improveddirectionality characteristics'that are substantially independent of thefrequency of the impinging sound waves.

Another objectof this invention is to improve the directionality ofunidirectional sound pick-up devices of the type having substantially acardioid response characteristic as describedin the foregoing.

and 1/4.

Another feature of the invention resides in a sound pick-up devicehaving adjustable means for setting the directivity index at any desiredvalue between 1/1 and 1/4.

A further feature of the invention involves a unidirectional transmitterhaving a plurality of axes of minimum sensitivity arranged symmetricallyabout the 180 degree position and making therewithany desired anglebetween 0 degrees and degrees.

Another feature of this invention includes the use of impedance meansassociated with the sound responsive means of a unidirectionaltransmitter for controlling the directivity index and the minimumresponse direction or directions thereof.

Other andfurther objects and features of this invention will beunderstood more clearly and fully from the following detaileddescription with .reference to the accompanying drawings in which: Fig.l is a wiring diagram illustrating one modification of this inventionFig. 2 is a sectional view of a transmitter embodying anothermodification of the invention;

Figs. 3 and 4- are respectively sectional and 'elevational views of amodified cover means for the device of Fig. 2 and which, when usedtherewith, embodies a further illustrative modification of theinvention;

Fig. 5 is a wiring diagram illustrating one way of connecting thetransmitter of Fig. 2 to an output circuit;

Fig. 6 is a plot on a decibel scale showing the directionalcharacteristics of a true cardioid this invention; and

Fig. 7 is a plot showing the relation between the directivity index anda factor representative of the relative outputs of the pressure andpressure gradient elements of a unidirectional transmitter made inaccordance with this invention.

Referring to Fig. 1 of the drawings, l and II designate sound pick-upelements responsive respectively to the pressure gradient and thepressure of incident sound waves. The elements l0 may be a bidirectionalpick-up device such as the so-called ribbon velocity microphone. Thepressure element l I may be a moving coil or other type of pressureresponsive transmitter or pick-up. The unidirectional transmitterdisclosed in application Serial No. 216,684 filed June 13,1938 byWilliam R. Harry "and Robert 'N. Marshall now Patent 2,227,580 issuedJan. '1, 1941, and comprising one each of such elements, is particularlysuitable.

The bidirectional element l0 may have its output connected to theprimary winding l3 of a transformer l2, which may have a secondarywinding I 4 and a tertiary winding l5. An inductor I6 and resistor l1may be connected across the terminals of transmitter element Ill. Theresistance represented by H may be included in the winding of theinductor l6 or the primary winding I3 of transformer l2 may be designedto include both of these impedances. A

resistor l8, capacitor is, and switch 20 may be serially connected tothe tertiary winding l5 of the transformer l2. The secondary winding 14of the transformer may be provided with a plurality of taps 2| to whichconnection may be contactor 22 set on a tap 2| to give the desiredcomplete cancellation at the 180 degree position.

The equation of a true cardioid such as curve A of Fig. 6 may be writtenE=7c (1+cos 0) (l) where E equals transmitter output, k equals aconstant of proportionality and 0 equals the angle of sound incidence.Curve A has for convenience been shown as plotted to a decibel scalerather than on the basis of the above equation.

The adjustment giving the foregoing curve has heretofore been consideredoptimum. However, in accordance with this invention it has been foundthat other adjustments oifer improvements in directional characteristicfor some conditions. If the sensitivity of the pressure gradient elementis held constant, and the output of the pressure element is adjusted-tosome value m, the

made" by means of switch arm or contactor 22. I

The transformer secondary l6 may be connected in series with thepressure transmitter element l I and to ouptut terminals 23 and 24 byconductors 25, 26 and 21. The secondary winding and hence thebidirectional transmitter element may be shunted by means of a switch28. A plurality of shunting means 29, 30, 3| and 32 may be connectedacross the output of the pressure element ll between conductors 26 and21. The shuntin elements may be connected in circuit by means ofswitches 33, 34, 35 and 36 or other suitable means such'as amulticontact switch. The shunt 29 is of very low resistance forshort-circuiting element II when desired. The resistors 30,3! and 32 mayhave different values for introducing more-or less attenuation into theoutput of transmitter H. The elements described in the foregoing may beconveniently housed in a single casing as indicated by the broken line31; thereby comprising a unitary, unidirectional transmitter or pick-updevice. This transmitter may have its terminals 23 and 24 connected toan-output transformer 38 and thence to a suitable amplifier representedby its first tube 33.

The impedances l6, I1 and I5, I8, l3 comprise output may be representedby E=k (m+cos 0) (2) .If then, m is adjusted to some value less than i,

the directional characteristic will assume a form different from acardioid, for example, that shown by curve B (plottedon a decibel scale)of Fig. 6. A reduction of m below 1 will have two efiects. First, therewill be two angles in each plane through the 0 degree-180 degree axis,symmetrically disposed with respect to the 180 degree position, forwhich the sensitivity is zero. Second, there will be an increase in thesolid angle efiiciency for sound of random incidence efliciencyf0r,sound of normal incidence which may be written 0 mode where r (o) issensitivity which is a function of the angle of incident sound 0. Forthe case of a microphone or transmitter where E=k (m+cos 0) it may beshown that this reduces to In' Fig. '7 m (plotted in decibels) and thedirectivity index have been plotted against the angles respectively lowfrequency and high frequency I equalizing means as is more fully pointedout in the prior Harry et al. application Serial No. 216,684. In orderto obtain as near as possible a cardioid characteristic over aconsiderable portion of the desired frequency range, the sound pick-upmeans just described is adjusted so that there is complete cancellationbetween the pressure gradient and pressure element for sound from the180 degree position or directly in the rear. The substantially truecardioid characteristic may be obtained by having switch 20 closed Whena transmitter of this type is used for sound reinforcement, such as inpublic address systems, the problem is somewhat more complex. The amountof reinforcement which may be employed is limited by singing, wheresound,

either direct from the loud-speaker or originatand switches 28, '33, 34,35 and 38 open, with ing at the loud-speaker and reflected by nearbyconditions, the optimum value m may be differsurfaces reaches thetransmitter. Under these ent from that which gives the'lowestdirectivity representative of the relative output of the two pick-upvelements Ill and II, in several ways. For the true cardioidcharacteristic obtained by the adjustment previously indicated, m=1. Thevalue of 172. may be reduced below. 1 by closing one or more of switches33 to 33, inclusive, to reduce the output of the pressure element Whenswitch 33 is closed to short circuit element II the value of m becomeszero and the figure 8 pick-up pattern characteristic of thebidirectional microphone or transmitter isfobtainecl. Byconnecting oneor more of the resistors 30, 3| and 32 in circuit, a pick-up patternsuch asrepresented by curve B of Fig. '6 will be obtained. The value ofm may also be adjusted by means of the taps 2| and contactor 22. In thiscase a tap is chosen that will make the output of the element It highwith respect to that of element Various combinations of the foregoingadjustments may be employedto secure the desired pick-up characteristic.Other equivalent output adjusting means may also be employed to controlthe relative contributions of the two sound re-.

sponsive elements to the total output. For example, a continuouslyvariable attenuator could be connected to the output of one or both ofthe pick-up elements In and H. Where a transmitter is desired having acertain fixed directivity index and/or direction of zero response, thebidirectional and non-directional transmitter elements may be designedto have the proper relative response .without the use of exteriorattenuating means.

The foregoing technique may be extended to produce similar modificationsof the directional characteristics of other types of unidirectionalmicrophones, for example, one of type shown in Fig. 2.

Referring to Fig. 2 there is shown a pick-up device which depends forits directivity upon the phase shift through an acoustical and mechanical network consisting of a resistance and a stiffness. The pick-updevice, as illustrated, is a condenser type transmitter. A stationaryelectrode 40 may be secured in a casing 4| having diaphragms 42 and 43attached thereto. Diaphragm 42, which may becalled the front diaphragm,is insulated from casing 4| by mounting means 44 of insulating material.The electrode 4|! and diaphragm 42 are included in the electricalcircuit of the transmitter but the back diaphragm 43 is not, itsfunction being acoustic only.

' Electrode 4||is provided with. a plurality of through orifices 45allowing communication between the front and back diaphragms'. Aplurality of orifices 46 passing only partly through the electrode 40provide acoustic impedance cavities.

The back diaphragm 43 is mounted on a ring member 41 which may beadjustably secured to casing 4|, as by meansof screwthread 48. Thisfeature allows an adjustment of the spacing between electrode 40 andback diaphragm 43 for a purpose to be presently set forth. 7 l

Electrical connection may be made to the transmitter by means ofterminals or binding posts 80 and ii. The terminal 60 makes electricalconnection to the electrode 40 through the casing 4|. A conductor 62connects .the diaphragm 42 to terminal 6|, which is insulated' from thecasing 4|. As shown in Fig.5, the

pick-up device or transmitter HI may be" associated with a source ofbiasing potential, such as battery H, and may be connected to suitableamplifying means represented by vacuum tube constitutesaphase shiftingnetwork. Thus, the.

The normal operation of this transmitter, that is, the operationproviding a substantially cardioid pick-up pattern, may be described asfollows:

Sound arising from the rear will, because of the difference in path,reach the back diaphragm somewhat before it reaches the front surface ofthe front diaphragm. The back diaphragm is normally spaced very close tothe electrode so that the viscosity of the air between them constrainsthe diaphragm to move in phase with the sound pressure. That is, thisdiaphragm functions as a series acoustic resistance. Atthe' same timethe volume of air in the orifices 45 provides a shunt compliance andthis in combination with the resistance contributed by the backdiaphragm phase of the sound pressure reaching the rear surface of thefront diaphragm is delayed. In the normal adjustment of the instrumentthis phase shift is made equal to the delay due to path differencebetween the front and rear .of the transmitter. Thus, the pressure willbe the same on 0th sides of the front diaphragm and there will be nomotion and hence no output. It is apparent that as the direction of theincident sound is changed the delay due to path difference will bereduced and the sensitivity of the transmitter will increase to amaximum for sound from directly in front. I

The phase shift of the network comprising the back diaphragm and theorifices 45 may be adjusted by variation of the compliance of the air inthe orifices 45, that is, by varying the volumecontained in theseorifices, and. by variation of the resistance supplied by the backdiaphragm.

ber and size of all of the orifices both 45 and 46. To obtain a modifieddirectional characteristic,

wherein the minimum sensitivity occurs at some angle other than 180degrees it will be necessa y to reduce the phase shift produced by theback diaphragm and the orifices 45 to an amount equal to the path delaydifference at the new angle of minimum sensitivity. This may be done byincreasing the compliance due to the orifices 45 or by decreasing theresistance due to the back diaphragm. The compliance of the orifices 45may be increased byincreasing their volume. The resistance may bereduced either by increasing the number of orifices 45 and 46 or byincreasing the separation of the rear diaphragm from the electrode 40.Any suitable combination of the above adjustmentmay be employed. Whereit is desirable to have the directional characteristics adjustable thescrewthreaded mounting ring 41 provides a convenient means for varyingthe spacing between the back diaphragm 43 and electrode 40.

A modification of this type of transmitter may be made by employing theresistance of a narrow slot in place of the resistance due to the backdiaphragm' '43. Such a resistance slot may be provided by meansillustrated in Figs. 3 and A cover member 49 adapted to replace themounting ring Won casing 4| may be provided with a slot 50. Anadjustable slide 5| mounted in guideways 52 may be provided for varyingthe eflective size and thus the resistance of slot 59. This slot whichwould be ordinarily very narrow in practice is shown somewhatexaggerated in width in the interest of clarity of illustration. Othersuitable means may be provided for ad- Justing the resistance of theslot 50. Variation of the size of the slot 50 will produce an effectequivalent to that obtained by changing the spacing between diaphragm 43and electrode 40. Ob-

viously, other means than those illustrated may be employed foradjusting the size of slot 50. The slot 50 may also be proportioned togive a particular, desired directional characteristic and no provisionmade for adjustment.

In devices of the type illustrated in Figs. 2, 3 and 4 a reduction ofthe resistance in the phase network will reduce the phase shift whichwill have an effect similar to the reduction of the value of m in thecombination type transmitter. The foregoing features as set forth withrespect to the transmitter illustrated in Figs. 2 to 5, inclusive, arenot limited to the condenser type structure shown. Other voltagegenerat-.

ing or modulating means may be associated with a sound responsiveelement such as diaphragm G2 and with appropriate acoustic impedance 6means to produce equivalent results.

Although the invention has been disclosed with reference to specificembodiments thereof, it will be understood that it is not restrictedthereto but is limited in scope by the appended 10 claim only. J

What is claimed is: A sound pick-up device comprising a ribbon velocitytransmitter, a moving coil pressure transmitter, a pair of outputterminals, a transformer having a primary ,winding and a secondarywinding, a plurality of resistors, and switching means associated withsaid resistors, said transformer having its primary winding connected tothe output of the velocity transmitter. the

secondary winding connected serially with the moving coil transmitterand the output terminals, said switching means serving to connect one ormore of said resistors in parallel with'said moving coil transmitter forreducing the output thereof, thereby improving the directionality of thepick-up device.

WILLIAM R. HARRY.

